1. Field of the Invention
The present invention relates to an aluminum alloy with good high-temperature strength and good abrasion resistance f or internal-combustion pistons, which is suitable for pistons to be used in internal-combustion engines such as diesel engines and gasoline engines, and also to pistons comprising the aluminum alloy.
2. Description of the Related Art
Hyper-eutectic Al--Si alloys that contain Si in an amount of not smaller than 12.6% by weight have a small thermal expansion coefficient and good abrasion resistance. While solidifying, the melt of such an Al--Si alloy produces primary crystals of pro-eutectic Si having high hardness. Therefore, the alloys are used for internal-combustion engine pistons that require high abrasion resistance. However, the machine workability of the alloys is poor since the growth of pro-eutectic Si therein is too great.
For effective utilization of energy resources, recently, it is desired to increase the combustion efficiency of internal-combustion engines. The increase in the combustion efficiency involves an increase in combustion temperatures, for which various parts that constitute the internal-combustion engines, especially pistons, require good high-temperature strength.
For conventional internal-combustion engine pistons, often used are eight types of aluminum alloys of JIS H5202 (AC8A, AC8B, AC8C). These are all Al--Si--Cu--Mg alloys, of which AC8A and AC8B additionally contain Ni. However, these conventional aluminum alloys have poor high-temperature strength.
Japanese Patent Publication (JP-B) Sho-60-47898 discloses an improved aluminum alloy which is prepared by adding V and/or Mo to an Al--Si--Cu--Mg alloy and which has good high-temperature strength while still having good castability intrinsic to the Al--Si--Cu--Mg alloy base.
Japanese Patent Application Laid-Open (JP-A) Hei-8-104937 discloses a method for improving both the high-temperature strength and the abrasion resistance of an Al--Si--Cu--Mg alloy by adding P, Ca, Fe and Ti to the alloy, in which the ratio of P and Ca to the other additives is controlled to fall between 0.5 and 50 by weight so that the action of P to produce fine pro-eutectic Si grains is protected from being attenuated by Ca and so that the action of Ca to improve the eutectic texture of the alloy is protected from being attenuated by P.
However, the techniques disclosed in these publications are still problematic in that the alloys proposed therein are not resistant to thermal loads to be applied to the proposed internal-combustion pistons, as their strength at high temperatures (especially, at 250 to 300.degree. C.) is poor, and that the thermal expansion coefficient of the alloys is large and the abrasion resistance thereof is poor, as the uppermost Si content of the alloys is limited to 13%.
Also known are nine types of aluminum alloys of JIS H5202 (AC9A, AC9B) having a low thermal expansion coefficient and improved abrasion resistance which, however, are still problematic in that their high-temperature strength is low and their castability and workability is extremely poor.